56 Prof. T. R. Jones and Mr. J. W. Kirkby on 
and a slight rim bounding the free margin: the surface, in 
unworn examples, is reticulately ornamented, and is usually 
marked with numerous rather fine longitudinal wrinkles, 
due to the thickening of the longitudinal walls of the net- 
work. 
The Upper-Silurian P. variolata, J. & H. op. cit. p. 418, 
pl. 13. f. 6, is a near ally ; but is shorter, has its sulcus more 
central, and shows only a pitted ornament. 
Some small bivalve carapaces from the Carboniferous strata 
of West Scotland t, and others from Shropshire, are allied to 
the form under notice, having suboblong outline, longitudinal 
wrinkles (stronger), and some fine reticulation ; but the sulcus 
is contracted to a central p7t, such as is found in some Primitic. 
We have also a small smooth form from Lanarkshire. 
4, Bairdia equalis, D’Kichwald. Pl. VI. fig. 4. 
We identify a specimen from the yellow limestone of Likh- 
wine with this species. It is 25 inch long, 7 inch high, 
smooth, swollen, of a subtrapezoidal outline, with the pos- 
terior extremity blunt, and with a strong dorsal and ventral 
overlap. 
5. Bairdia ampla, Reuss. Pl. VI. fig. 5. 
Two very fine examples of a Bairdia sent us by M. d’Eich- 
wald as B. curta, from Sloboda, in the Government of Toula, 
appear to us to belong to B. ampla, Reuss, known in both the 
Carboniferous and Permian formations. 
The perfect shape of B. curta, M‘Coy, has been given by 
one of us in plate 61. fig. 1, ‘ Monthly Moet Journ.’ vol. iv. 
1870. 
6*. Bairdia plebeia, Reuss, var. rhombica, Jones. 
PV hes 6: 
Included with the specimens of the last species is a single 
example of what seems to be, if not a distinct species, a 
rhombic variety of B. plebeia, described and figured in the 
‘Trans. Tyneside Nat. Field-Club,’ vol. iv. 1859, p. 42, pl. 11. 
figs. 10, 11,12. B. plebeva is common in both the Carboni- 
ferous and Permian formations ; var. rhombica is Permian also. 
+ “Kirkbya scotica,” J. & K. MS., ‘Trans. Geol. Soc. Glasgow,’ vol. iii. 
Supplem. Carb. Foss. p. 28. Unless specimens with concentric longitu- 
dinal ribs occur, this species will have to be allocated to Primitia. 
